The present invention is related to antennas, and more particularly to array antennas employing dual polarized antennas having oppositely oriented spiral like antenna elements.
High gain antennas with circular polarization are useful for communication purposes as well as radar and other receiving and transmitting uses. Typically dual circular polarization for a single broadband antenna element is achieved by employing sinuous antenna elements or modulated multi-width spirals. In both cases, the elements are fed at nearly the same point in space thereby increasing the complexity of the feed. The sinuous antenna is planar, broadband and dual polarized from a single aperture. However, the sinuous antenna has several drawbacks, not the least of which is that it is difficult to construct. The sinuous antenna includes at least four separate antenna arms on its planar surface. The antenna arms radiate out in identical sinuous patterns symmetrically about a center point. The antenna arms cannot contact each other, and each antenna arm must be center fed independently of the others. Given the close proximity of the centers of the arms, the design does not lend itself to low cost manufacturing schemes. This is further complicated by the fact that the ability of such antennas to receive or transmit high frequency signals is determined by the accuracy of the antenna arms near the center of the antenna close to the feed point. Accordingly, as high accuracy is required of the centers of the separate antenna arms, and each antenna arm must be center fed, construction constraints necessarily either diminish the high end abilities of sinuous antennas and/or make construction of sinuous antennas more difficult and costly.
Further, sinuous antennas need additional circuitry, in the form of a hybrid circuit connected to the center feeds, to receive right-hand and left-hand circularly polarized signals. This additional hardware adds to the cost of the antenna, and requires additional manufacturing steps. Therefore, the sinuous antenna is complex and difficult to construct.
Another dual circular polarization antenna is disclosed in U.S. Pat. No. 5,416,234, which discloses an antenna having an upper set of spiral arms 10 and a lower set of spiral arms 12 which are oppositely oriented and stacked, as shown in FIG. 1A. This antenna allows for a dual polarized signal without the need for sinuous antenna arms and additional hybrid circuitry. While this allows less hardware, and thus eases the manufacture of the antenna as compared to a sinuous antenna, the elements are stacked directly above and beneath each other, and can be fed from the center of each element with feeds 14, as shown in FIG. 1B. This co-location of feed points makes manufacture of the antenna difficult.
Alternatively, the elements may be fed from ends of the spiral arms with feeds 16, as shown in FIG. 1C. While this configuration allows for more ease of manufacture as the feeds are not co-located, it does not allow for the elements to be conveniently arranged into an array, as the ends of the arms can not be connected to one another and the elements cannot be tightly packed within a lattice to support high frequency performance and still exhibit good low frequency performance. Also, the number of feed points for this arrangement would be increased, as each spiral arm in an element would need an individual feed point at the end of the arm. Further, the bandwidth is limited for end fed elements as compared to center fed elements.
Additionally, as the elements are stacked directly above and beneath one another, this can create coupling between the elements, thus degrading the signal from the elements. For a right handed circularly polarized signal and a left handed circularly polarized signal sent simultaneously, the location of these two elements may create coupling which can degrade the isolation between the two polarizations. Maximum isolation between the two polarizations is desirable. However, it must be accomplished without compromising dual circular polarization performance.
In accordance with the present invention, a dual array antenna is disclosed. The antenna has a first array comprising a first plurality of spiral-like antenna elements interconnected together on a first surface. The antenna has a second array comprising a second plurality of spiral-like antenna elements interconnected together on a second surface. Each of the elements has a feed point, with the feed points of the elements of the first array being offset from the feed points of the elements of the second array. In a preferred embodiment, the first and second surfaces are top and bottom surfaces of a substrate.
The feed points of the elements within the first array are separated by a lateral distance. The feed points of the elements of the second array are separated from the feed points of the elements of the first array by an offset lateral distance that is parallel to the lateral distance, and an axial distance that is perpendicular to the lateral distance. In one embodiment, the offset lateral distance is at least one-eighth of the lateral distance and preferably about one-half of the lateral distance. The feed points of the elements in each array lie in a plane, with the plane for the first array being parallel to the plane for the second array, and the elements in the first array are offset in at least an X direction from the feed points of the elements in the second array, and preferably offset in both the X direction and Y direction. The feed points are connected to baluns which are non-co-planer relative to the first plurality and second plurality of antenna elements. In one embodiment, the antenna also has one or more tuning members disposed outwardly of the substrate member.
The offset of the feed points of the first and second arrays of antenna elements is determined based on a number of factors. These factors include: impedance associated with the antenna, an operating frequency range associated with the antenna, a bandwidth associated with the antenna, and a gain associated with the antenna.
The antenna elements include a loop portion having a free end, and a tail portion. On one embodiment, the tail portions of adjacent antenna elements within the first or second array are joined together and are substantially straight where they are joined together.
Based on the foregoing summary, a number of advantages of the present invention are noted. A dual array is provided that can generate left hand circularly polarized and right hand circularly polarized signals. These signals are generated with reduced coupling between the two arrays, thus enhancing the isolation of the two polarizations. The antenna can also generate or receive high frequency signals. The configuration of the antenna allows for ease of manufacturing, thus reducing cost associated with the manufacture of the antenna.
Other features and advantages will be apparent from the following discussion, particularly when taken together with the accompanying drawings.